Replacing A Double Light Switch

[terry]

Hey folks. I'm a diy'er lookin for some advice.

I'm replacing a volex double light switch with an antique brass plate. Problem is that when turned over the volex has 4 terminals on top and 4 on bottom however the brass plate has 3 on top and 1 on bottom. Is this the correct plate and do I have to more homework on the subject?

On a similar vane. Can I use a 45 amp antique brass cooker switch to replace a volex shower switch with what iv been advised is the same amps?

Any useful advice welcome

Sorry. Replacement has 3 terminals on top and four on bottom.

 

[ProDave]

Shame new members cannot post pictures.

NO 1 THING.  Do NOT disconnect any wires from your old switch yet until you have taken a photo or drawn a diagram of how it's wired now.  If there are more than 1 wire of each colour, label them in some way e.g with tape so you can always get it back to exactly how it was before.

Now tell us clearly in text (as you can't post a photo) the terminal marking and what wire goes into which terminal. Treat left and right switch separately as they are two completely separate switches (even though they may be the same lump of plastic)

I have to say 4 terminals per switch sounds wrong as that would suggest you have two intermediate switches which is very very unusual.

The usual difference is some switches put all the switches the same way up, so I would expect 4 terminals at the top and 2 at the bottom, and some put alternative switches the other way up, so I would expect 3 terminals top and bottom.  What you have does not match either of those.

 

[terry]

Cheers for your reply. Yeah I got that wrong, thought an extra wire was a terminal. I would definitely mark each before disconnecting as iv seen what happens when you dont.

iv done some internet studying last night so this dont look as scary as before. (Famous last words!).

okay, heres what I have. Current switch has l1 l2 and l3 connections. All used in the first switch which controls the hall and only l1 and l2 used in the second switch which controls the bathroom light. That would be an easy switch if both had the same but the new switch has a common port so how do I tell which switch on the existing socket is the common port. L2 in both current switches has a yellow wire and l3 in the hall switch has a blue wire.

Just need to find the common and im good to go. I think.

 

[ProDave]

Simple

Old switch     New switch

L1                 C

L2                 L1

L3                 L2

Just label up all the wires so you know which is which first in case you get it wrong.

And when you come to fit the new switch, one half of it will appear to be upside down compared to the old switch.

 

[terry]

That sounds easy enough. So would l1 if not marked so always be the common wire?

Also I seen on you tube etc that it dont matter where the other wires go as long as they correspond. Eg. L1 to l1 or move to l2 to l1 etc. Is that right as long as common wiring is correct?

I'll switch the plates around tomorow an let you know how I get on.

the next job I have is: switching a volex shower switch for an antique brass cooker switch. They both have an indicator light. Would this be okay to use for thw shower. The switch aint in the bathroom, its in the hallway outside the bathroom. Would it be okay to use this if they are both 45 amp. I assume the current volex shower switch will have the amps clearly marked when I take the plate off? The only other choice was a water heater switch which was around 20amps or so. A friend said go for the 45 amp plate. (Im still waiting on the plate being delivered so dont know much of how it looks)

 

[ProDave]

Each individual switch will have one terminal at one end, and two terminals at the other end.

the single one is the common.

One VERY important thing, I have just noticed the new switch is brass. there should be an earth terminal on the front of the switch plate. Make sure you connect the earth wired to that. they may not be connected anywhere at the moment if it's a plastic back box.

 

[terry]

Yeah I noted the earth topic on the other vids. I need to pick up some earth wire for the single plate I fitted the other day, the plastic ones themselves werent earthed but the inner box has an earth connection on it so was gonna attach the earth from the new plate to that.

what exactly is the earths purpose and what is the risks of not having it. I mean I know its electricity and shock is the risk but how does it happen.

Btw. Thanks for all your advice mate. Really helpful.

 

[ProDave]

In case some part of the switch goes faulty and shorts to the metal plate, having it earthed will inure it trips the circuit.

An earth fly lead from the earth terminal in the back box is fine.

 

[terry]

Cool. Thanks for you help mate. Ill post how it goes ( or doesnt) tomorrow.

 

[brman]

In case some part of the switch goes faulty and shorts to the metal plate, having it earthed will inure it trips the circuit.

An earth fly lead from the earth terminal in the back box is fine.

All good advice but I think you forgot something Dave

Terry, please check that the earth terminal on the backbox is actually connected to earth.

1) does the cpc (circuit protective conductor or earth to most people) of the supplying cables terminate there?

2) Does a check with a meter (peferably a MFT but a multimeter will do if it must) confirm a low resistance (less than a few ohms) to the main earth terminal? (the main earth terminal is (normally) the earth at your fusebox)

You have probably done 1) already, but 2) is equally important. It is actually fairly common to find the cpc in a twin and earth is not actually connected to earth. It it also pretty common to find it doesn't even have one on  lighting circuits.

btw. Do you know if you have a RCD protecting this circuit? If not I might withdrawn my comment that a multimeter is good enough to check the earth.

You might think this sounds a bit unecessary for just changing a lightswitch but if you get it wrong a fault can kill you and it is actually pretty common to trap a live conductor between a switch and its backbox when fitting it, creating a short.

 

[terry]

Hey guys. Cheeers for the advice.

the plates have been replaced (picking up the shower switch later to replace that) and also fitted an earth from the new plate to the brass replacement switch to the metal inner case)

One thing though. I dont have a multimetre to test the earth and ohms and wouldnt know how to do that if I did tbh. The earth from inside the case appears to run back through with the other wiring and I would assume they are connected?!!

The wylex fuse box is marked like this: sockets, sockets, cooker, (rcd protected test switch) smoke alarm, lights, lights (main switch for all circuits-not rcd protected.

when testing the rcd the lights stay on so would this mean they are not rcd protected?

Is this a big job to have this rcd fitted and what would it involve? Obviously I wouldnt attempt this myself. I do have another netal light in the kitchen installed before we moved in also earthed to the backplate.

 

[roys]

Terry don't see the shower feed mentioned on your list from the Wylex board, does the shower perhaps have it's own seperate unit with a RCD and Circuit breaker?

 

[terry]

Hi Roy.

its a relatively ne build flat. I would assume the shower does have an rcd. Mind I dont know an rcd from a dvd but the first fuse box does have a clearly marked rcd with test switch. The shower has its own fuse on the second wylex fuse box (not the same box as the lights and sockets) The lights have a white switch on their fuse, the sockets and cooker have a red switch (and clearly marked rcd) the shower has a separate fuse with red switch but there is no clearly marked rcd as seen on the first box.

each set of fuses on each box apart from the sockets and cooker has a main large red switch separate from the fuses.

Hope this makes some sense.

 

[roys]

An RCD will have a test button on it and will normally have something like 30mA written on it.  Can you upload a pic (you have over 10 posts now).

 

[terry]

Not sure how to upload a pic, dont see an option for that.

there seems to be only one rcd in the first fuse box and when tested turns off all sockets and cooker. Everytjing else has its own fuse and main switch. Doesnt the main switch trip when a fault occurs? And if so, whats the purpose of an rcd? (Just so I get a better undersatnding)

Not sure how to upload a pic, dont see an option for that.

there seems to be only one rcd in the first fuse box and when tested turns off all sockets and cooker. Everytjing else has its own fuse and main switch. Doesnt the main switch trip when a fault occurs? And if so, whats the purpose of an rcd? (Just so I get a better undersatnding)

 

[NozSpark]

Basically....

A main switch is just a switch

A MCB (circuit breaker) trips on overload

and a RCD trips in the event of a fault

A RCD also has a "test" button on it which should be pressed every 3 months

 

[terry]

Not sure how to upload a pic, dont see an option for that.

there seems to be only one rcd in the first fuse box and when tested turns off all sockets and cooker. Everytjing else has its own fuse and main switch. Doesnt the main switch trip when a fault occurs? And if so, whats the purpose of an rcd? (Just so I get a better undersatnding)

 

[brman]

dealing with the light circuit first, the fact it has an earth in the cable and it is connected at the switch end is good news. It will probably be connected to the main earth properly but that is  just a guess. The only way of telling is the test it.

This is down to your assessment of risk. As an electrician I would test it as there is no way I would want to leave an installation potentially dangerous. As a DIYer you are probably the same as 99% of all other DIYers who don't know the risk and so don't check it.

It is entirely up to you whether you get someone to test it or take the risk.......

I would not worry over much about a lack of an RCD on the lighting circuit. This is pretty standard with properties wired to previous version of the regulations. But given you don't it is more important that have a proper connection on the earth as you really need a fault to go to earth, not through you!

btw. In case you don't know, an RCD will trip on earth leakage of about 1/200th what the lighting fuse/MCB will trip on. ie the RCD has a good chance of saving your life it you touch something live, a fuse/MCB is pretty unlikely to trip so you will just fry......

This is the thing with electrics. 90% of the time a compentent DIYer can do it with without any testing and never have a problem so there is a misconception you can just wire things up and that is it. Unfortunately the other 10% isn't noticed until someone electrocutes themselves, a house burns down or (if you are lucky) an electrician doing some other work finds the problem.

As to the shower RCD (or not) a picture is definitely the easiest way to go

 

[terry]

I seemed to have pressed 'post' more than once on that last reply. Goin on what you say, I have only one rcd thats connected to sockets and the cooker. Everything else doesnt have one. Mind you the first box has a label that states part is protected by rcd. Nothing like that on the fusebox for the shower. So whats the next step, have an electrician connect an rcd to the lights and is that absolutely neccessary. Shower? Should that switch have an rcd and if there are regulations for these things then why would the original building contractors not have fitted these?

 

[brman]

btw. I am not sure how to upload pics either as I normally host them elsewhere and link to them.

But if you click on the "more reply options" button you get a more complex reply form with an "attach files" section at the botton. You could always attached a jpg that way I assume.